Thermal convection of liquid sodium in inclined cylinders

The effect of inclination on the low Prandtl number turbulent convection in a cylinder of unit aspect ratio was studied experimentally. The working fluid was sodium (Prandtl number Pr = 0.0094), the measurements were performed for a fixed Rayleigh number Ra = (1.47 +/- 0.03) x 10(7), and the inclination angle varied from beta = 90 degrees (the Rayleigh-Benard convection, the temperature gradient is vertical) up to beta = 90 degrees (the applied temperature gradient is horizontal) with a step Delta beta = 10 degrees. The effective axial heat flux characterized by the Nusselt number is minimal at beta = 0 degrees and demonstrates a smooth growth with the increase of the cylinder inclination, reaching a maximum at angle beta approximate to 70 degrees and decreasing with a further increase of beta. The maximal value of the normalized Nusselt number Nu(beta)/Nu(0) was 1.21. In general, the dependence of Nu(beta) in a cylinder with unit aspect ratio is similar to what was observed in sodium convection in inclined long cylinders but is much weaker. The structure of the flow undergoes a significant transformation with inclination. Under moderate inclination (beta less than or similar to 30 degrees), the fluctuations are strong and are provided by regular oscillations of large-scale circulation (LSC) and by turbulence. Under large inclination (beta > 60 degrees), the LSC is regular and the turbulence is weak, while in transient regimes (30 degrees < beta < 60 degrees), the LSC fluctuations are weak and the turbulence decreases with inclination. The maximum Nusselt number corresponds to the border of transient and large inclinations. We find the first evidence of strong LSC fluctuations in low Prandtl number convective flow under moderate inclination. The rms azimuthal fluctuations of LSC, about 27 degrees at beta = 0 degrees, decrease almost linearly up to beta = 30 degrees, where they are about 9 degrees. The angular fluctuations in the vicinity of the end faces are much stronger (about 37 degrees at beta = 0 degrees) and weakly decrease up to beta = 20 degrees. The strong anticorrelation of the fluctuations in two halves of the cylinder indicates the torsional character of LSC fluctuations. At beta = 30 degrees, the intensity of the oscillations at the periphery of the cylinder falls sharply to the level of oscillations in the central plane and the anticorrelation disappears; the torsional fluctuations vanish.